The present invention relates to a micro electron gun and a flat display apparatus with an array of micro electron guns as well as methods of their manufacture.
In receivers for television, and monitor displays for personal computers, it has been traditional to use either a CRT (cathode ray tube) set or a LCD (liquid crystal display).
A CRT is made up of an electron gun, a deflector for sweeping an electron beam projected from the electron gun and a fluorescent screen that illuminates when it has an electron beam impinging thereon. The electron gun is formed, e.g., of a filament made of a resistor such as a tungsten wire, and a focusing coil for focusing thermoelectrons emitted from the filament.
Designed to heat the filament by passing electric current through it to emit thermoelectrons, an electron gun of this type is poor in energy efficiency for extracting the electrons into a required beam, since a portion of the electric energy is consumed to emit light and heat. Such electron guns are also deficient in safety, because they entail a high voltage and an elevated temperature.
An electron gun of field emission type is also available in which a high voltage is applied to a pointed metal or semiconductor and an electron beam is extracted using an electric field concentrated at this pointed end. Energy efficiency is poor here, too, because of the need for a high voltage.
Because the deflector deflect the direction of travel of an electron beam by an electromagnetic force, it must be capable of attaining greater angles of deflection for corresponding to a large fluorescent surface, namely the screen, so it is necessary that the deflector has a large volume in size or applied in a big electrical power.
Thus, the electron guns in the CRT system are as poor in energy efficiency as 0.01% (in quantum efficiency as 0.1%), and are hence unsuitable for a power saving apparatus. Further, in the principles of their operation they can not be made thin, and hence it is impossible for them to be made flat such as for a wall type television.
In order to solve the problems of the CRT system, the LCD device has come to spread widely. The LCD device, however, has left the problems mentioned below.
The LCD device is made up of a backlight panel as a light source, a liquid crystal and a polarizer interposed between substrates which have electrodes for controlling the light transmittance of each of the pixels, a color filter, and a drive circuit for applying voltages to the electrode.
In any LCD device of this type, the attempt to raise its brightness requires raising the brightness of the backlight panel light source, but this entails raising the brightness of pixels that are not required to raise its brightness. Hence, poor energy efficiency ensues here. Further, the backlight panel light source, which is made up of fluorescent tubes or a LED array panel or an EL panel, is hard in the principles of its operation to obtain as high a brightness as in the CRT system.
Also, in recent years there has been proposed an image display system which by using an electron gun for each of the pixels can be made as thin as the LED system and provides a brightness as high as with the CRT system. The electron gun in this system is an electron gun of field emission type having a pointed area for each pixels, and the display is constituted by these pixels which are formed on, e.g., the surface of a semiconductor substrate by machining the semiconductor surface. In such a system, it is technologically difficult to machine so that a pointed area may be formed for each of the pixels, and there is also the problem that even if being effective the electric field concentration of the pointed area, even high voltage is needed in order to extract electrons from the semiconductor. For these reasons, the system has not yet been put to practical use.
In sum, the conventional image displays as mentioned above leave the problems unsolved that they are poor in energy efficiency, incapable of being made thin and much unsatisfactory in brightness.
With those problems taken into account, it is an object of the present invention to provide an electron gun of quantum size effect type which by using a quantum size effect makes it possible to extract electrons easily from a semiconductor and can be allocable as each of pixels.
It is another object of the present invention to provide an image display apparatus using such electron guns that is high in quantum efficiency, high in brightness and small in thickness.
It is a further object of the present invention to provide methods of making an electron gun of quantum size effect type and an image display apparatus as mentioned above.
In order to achieve the objects mentioned above, as set forth in claim 1 and in the appended claims there is provided in accordance with the present invention a micro electron gun of quantum size effect type, characterized in that it comprises a semiconductor substrate and an electrode; and a layer or layers of micro particles having a quantum size effect formed between the said semiconductor substrate and the said electrode whereby an electron conducting between the said semiconductor substrate and the said electrode has a longer mean free path. It is made possible thereby to accelerate an electron under an electric field so as to impart thereto an amount of energy not less than the work function of a substance constituting the electrode, and to take out an electron from the electrode into vacuum.
As set forth in claim 2 in the appended claims, the invention further provides a micro electron gun of quantum size effect type, characterized in that between the said semiconductor substrate and the said electrode the said quantum size effect micro particles are so formed in the said layer or layers that they increase in particle size gradually from the said semiconductor substrate towards the said electrode. The micro electron gun is thereby made operable even with an applied voltage further reduced.
An aforesaid micro particle having a quantum size effect is preferably a quantum size effect micro particle made up of a single crystal semiconductor micro particle having a particle size in a nanometer order and an insulating layer having a thickness in a nanometer order and with which the said single crystal semiconductor micro particle is covered. This makeup permits establishing discrete energy levels as a quantum size effect while making it possible for an electron to tunnel.
The said single crystal semiconductor micro particle is preferably a silicon single crystal micro particle, and the said insulating layer with which the same is covered is then formed of either a silicon oxide or a silicon nitride film.
The present invention also provides in another aspect thereof a flat display apparatus characterized in that it comprises a planar array of micro electron guns of quantum size effect type and a fluorescent plate mounted above the planar array of the said micro electron guns.
An aforesaid micro electron gun of quantum size effect type for use in the flat display apparatus comprises a semiconductor substrate and an electrode; and a layer or layers of micro particles having a quantum size effect formed between the said semiconductor substrate and the said electrode whereby an electron conducting between the said semiconductor substrate and the said electrode is supplied with an increased mean free path.
Preferably, in the said micro electron gun of quantum size effect type, the said quantum size effect micro particles are so formed in the said layer or layers between the said semiconductor substrate and the said electrode that they increase in particle size gradually from the said semiconductor substrate towards the said electrode.
The flat display made to include the planar array of electron guns and the fluorescent plate may comprise a plurality of lower electrodes made of semiconductor and arranged in a form of stripes; a layer or layers of quantum size effect micro particles as aforesaid formed on the said lower electrodes; a plurality of upper electrodes formed on the said layer or layers of quantum size effect micro particles and arranged in a form of stripes extending orthogonally to those forming the said upper electrodes, wherein respective regions of intersection of the said upper and lower electrodes (as seen through from top or bottom) together with portions of the said layer(s) of quantum size effect micro particles interposed between the said respective regions of intersection make up the said arrayed micro electron guns for pixels whereby a selected one of the said pixel may be illuminated when a driving voltage is applied across a particular pair of the said upper and lower electrodes correspondingly selected.
The present invention further provides in yet another aspect thereof a method of making quantum size effect micro particles, characterized in that it comprises the steps of: introducing silane into a VHF-band plasma of argon to form Si single crystal micro particles; forming on a substrate layers of such Si single crystal micro particles so formed; and converting respective surface areas of the said Si single crystal macro particles in the said layer on the said substrate in the presence of a gaseous atmosphere into insulating films.
Here, the said Si single crystal micro particles may be formed by forming SiH2 radical, SiH3 radical and SiHn+ ions (where n=0 to 3) in the said VHF-band plasma of argon; forming nuclei of the Si single crystal micro particles from the SiH2 radical; and bonding the SiH3 radical and SiHn+ ions (where n=0 to 3) to the said nuclei to cause the latter individually to crystallographically grow, thereby forming the Si single crystal micro particles.
The step of forming a layer of Si single crystal micro particles may also include causing the Si single crystal micro particles to diffuse following a concentration gradient thereof, followed by their arrival to and deposition on the said substrate, forming there said layer thereof.
Also, the insulating layers on the surfaces of the Si single crystal micro particles may be formed by exposing the said Si single crystal micro particles to a gas atmosphere of O2 or N2 or to a gas plasma of O2 or N2.
Further, layers of Si single crystal micro particles may be formed by repeating the aforementioned process steps for making a layer of Si single crystal micro particles.
In the methods mentioned above, it is also possible to control the particle size of the said Si single crystal micro particles by controlling the time duration in which silane is introduced into the said VHF-band plasma of argon.
It ought to be noted here that when a voltage is applied across a semiconductor crystal, a conduction electron in the crystal is accelerated under the resulting electric field to gain energy. Thus, taking out an electron from the crystal into a vacuum may make it sufficient to accelerate the electron so as to impart thereto an amount of energy not less than the work function of the crystal. In a material where an electron undergoes scattering by an impurity atom, a lattice defect or a phonon, however, it is not possible to accelerate an electron over a distance exceeding its mean free path.
While the mean free path can be increased by lessening impurity atoms and crystal defects, it is not possible to increase the mean free path against the phonon scattering which is based on the crystal structure itself.
By the way, in a Si single crystal in which impurity atoms and crystal defects can be neglected an electron is given a mean free path of about 50 nm. Accelerating the electron over a distance of this mean free path of 50 nm so that it gains, e.g., 50 eV, of energy that is the work function of gold requires a field strength of 108 V/m, which comes to exceed an avalanche breakdown voltage of 3xc3x97107 V/m of Si single crystal. It follows, therefore, that an electron cannot be taken out from Si single crystal and, indeed, not only from Si semiconductor but from any known semiconductor crystal.
It may be noted in passing that the phenomenon that an electron is scattered by a phonon is a collision process in which both energy and momentum are conserved. To wit, a collision may occur between an electron lying at one of the energy levels of the conduction band and a phonon as a quantized lattice vibration having an energy of about KT (where K is the Boltzmann""s constant and T is the absolute temperature). Then, the electron upon either absorbing, or emitting a phonon may make transition either to an energy level in the conduction band that has the energy being added the energy of the phonon to the energy of the electron before the collision, or to an energy level in the conduction band that has the energy being subtracted the energy of the phonon from the energy of the electron before the collision. Also with the momenta being conserved as well, the electron changes its direction of motion depending on the direction in which it impinges the phonon.
The collision process to occur in this manner requires an empty energy level that corresponds to the energy of the conduction electron after the collision. The phonon""s energy being about KT, it is necessary that there exist such an empty energy level for the conduction electron in a neighborhood of about KT above or below its energy level before the collision.
In a crystal of ordinary size, energy levels in the conduction band are distributed almost continuously and densely, and the existence of an empty energy level for a conduction electron after collision causes a collision process to occur between the conduction electron and a phonon, and thus the conduction electron to be scattered by the phonon.
By the way, it is well known that an electron confined in a potential well has its discrete energy levels made higher as the width D of the potential well is made smaller; hence the difference between adjoining energy levels is then made larger as well. This effect is herein referred to as xe2x80x9cquantum size effectxe2x80x9d.
Thus, making the width D of a potential well small makes it possible to utilize its quantum size effect and in turn to derive an energy level width (difference) that is greater than KT. It is then made possible to increase the mean free path of an electron to an extent that it is no longer scattered by a phonon.
The present invention is therefore predicated on the use of this effect whereby electrons in a semiconductor in the course of their acceleration under an electric field are supplied with an increased mean free path so as to acquire an amount of energy not less than the work function of a material and thereby taken out thereof into a vacuum.